Busbar, method for manufacturing the same, and power module including the same

Abstract

A conductive busbar (2, 4) suitable for a semiconductor power module (8) is provided. The busbar (2, 4) comprises: a main board (210, 410), one or more legs (220, 420) extending from the main board (210, 410), and one or more legs (230, 430) formed at the free ends of the legs (220, 420). According to the present invention, an intersection line (L) between at least one of the legs (220, 420) and an associated leg (230, 430) forms an offset angle (α) relative to a longitudinal direction (X) of the main board (210, 410).

Description

[0001] The present invention relates to the field of semiconductor power modules, and in particular to a busbar used in such a power module.

[0002] A semiconductor power module including one or more semiconductor switches (such as IGBTs, MOSFETs or other power control semiconductors) is connected to a power input, a power output and one or more control connectors through which the switching of the semiconductors is controlled. The packaging of the power module is completed by some form of housing that protects the circuits and components from environmental influences.

[0003] A typical power module includes a base plate to which a substrate such as a direct bonded copper substrate (DBC) is mounted. On the top side of the substrate may be printed or etched a circuit pattern on which components such as power semiconductors are mounted. In certain high power devices, it is convenient to use busbar structures to conduct current in and out of the module between external contacts and connection points on the substrate.

[0004] Typically, such a busbar structure can be formed by stamping or cutting from a flat sheet of conductive metal such as copper and then bending it into a structure that can conduct current between the external contacts and the appropriate points on the substrate. Typically, such a structure has legs and feet, the legs connecting the upper plate-like structure of the busbar down to the horizontal plane of the substrate, and the feet forming a connection area parallel to the substrate. These feet are used to achieve a low resistance connection to the substrate by, for example, brazing, sintering or ultrasonic welding.

[0005] In use, such a module will be subject to varying electrical loads and the components mounted on the substrate will generate heat. Although most of this heat is usually conducted away, the temperature of the substrate and the connected busbars will vary. The problem arises because the thermal expansion of the substrate and busbars can be very different due to the fact that their coefficients of thermal expansion (CTE) are different. The substrate is usually a laminate with a ceramic core and thin conductive layers on each side. The busbars, on the other hand, can be solid copper or a laminate with several layers of copper connected by insulating films. The typical CTE of the substrate is 7 × 10 -6 / °C, while the typical CTE of copper is 16 × 10 -6 / °C.

[0006] This difference in thermal expansion coefficients results in a difference in linear expansion between the substrate and the busbar. This in turn results in stresses at the connection point between the busbar leg and the substrate. These stresses can in turn result in cracking at this interface and ultimately in disconnection. This process severely limits the reliability of this construction and is a significant disadvantage.

[0007] Therefore, an object of the present invention is to provide a busbar, a method for manufacturing the busbar and a power module including the busbar which can overcome or at least alleviate the above-mentioned drawbacks.

[0008] According to one aspect of the present invention, a conductive busbar suitable for a semiconductor power module is provided, the conductive busbar comprising: a main board, one or more legs extending from the main board, and one or more legs formed at the free ends of the legs, wherein the intersection line between at least one of the legs and the associated legs forms a certain offset angle relative to the longitudinal direction of the main board. As used herein, "conductive busbar" should refer to a busbar for conducting electricity.

[0009] In some embodiments, at least one of the legs of the busbar according to the invention comprises a twisted portion, by which the plane of the leg is rotated by an offset angle. In this way, the associated foot is also rotated by an offset angle relative to the case where the leg and foot are formed by simply cutting and bending a flat metal sheet.

[0010] In some embodiments, the offset angle formed by the intersection line between at least one of the legs and the associated foot relative to the longitudinal direction of the main board can be from 3° to up to 180°, for example, 5°, 10°, 15°, 30°, 45°, 60°, 75°, 90°, 105°, 120°, 135°, 150°, 165° or 180°.

[0011] In some embodiments, each leg of the busbar according to the present invention comprises a twisted portion, through which the plane of the leg is rotated by an offset angle. In this case, it is preferred that all of these legs are twisted in the same way (e.g., at the same angle and in the same direction).

[0012] In some embodiments, the legs of the busbar according to the present invention extend from two opposite sides, preferably two longitudinal sides, of the main board of the busbar. In this case, the legs on the two opposite sides of the main board of the busbar can be arranged symmetrically. In other embodiments, the legs of the busbar according to the present invention extend from one side, preferably a longitudinal side, of the main board of the busbar.

[0013] According to another aspect of the present invention, a busbar system is provided, which comprises two or more busbars according to the present invention, wherein the busbars are in close proximity with an air gap or an insulating film or layer therebetween.

[0014] According to another aspect of the present invention, there is provided a power module comprising the busbar or busbar system defined above.

[0015] According to another aspect of the present invention, there is provided a method for forming a busbar according to the present invention, the method comprising the following steps:

[0016] stamping or cutting a planar structure having a main plate and at least one leg from a conductive sheet;

[0017] bending the at least one leg out of the plane of the main board and bending an end portion of the at least one leg to form a foot; and

[0018] The at least one leg is twisted in such a way that an intersection line between the at least one leg and an associated foot forms an offset angle with respect to a longitudinal direction of the main board.

[0019] These and other aspects and advantages of the present invention will become apparent and readily understood from the following description of preferred embodiments with reference to the accompanying drawings, in which:

[0020] Figure 1 is a perspective view of a single-layer metal busbar known in the prior art;

[0021] Figure 2 Shown from Figure 1 A detail of one of the legs of the busbar, the leg being connected to an electrical contact of the substrate;

[0022] Figure 3 is a similar view of a foot of a busbar according to the invention, with Figure 2 The legs are twisted about 90° compared to the legs shown;

[0023] Figure 4 is similar to Figure 1 A perspective view of the busbars shown, with Figure 1 The legs are twisted about 90° compared to the legs shown;

[0024] Figure 5 is a perspective view of an alternative form of busbar known in the art;

[0025] Figure 6 is similar to Figure 5 A perspective view of the busbars shown, with Figure 5 The legs are twisted about 90° compared to the legs shown;

[0026] Figure 7 Is caused by Figure 4 A perspective view of a double-layer busbar system formed by two busbars with similar busbars and pivoted feet;

[0027] Figure 8 is a partial cutaway perspective view of a power module, in which a Figure 7 Double-layer busbar system;

[0028] Fig. 9 is a plan view looking down on the leg structure and showing the three possible positions of the rotated legs;

[0029] Fig.10 is a plan view looking down on the leg structure and showing two possible positions of the rotated legs;

[0030] Fig.11 is a flow chart showing the steps of a method for forming a busbar according to the present invention.

[0031] In the following we will refer to Figures 1 to 11 The technical solution of the present invention is described in more detail by way of embodiments. The description of the embodiments of the present invention with reference to the accompanying drawings is intended to explain the overall inventive concept of the present invention, but should not be construed as limiting the present invention.

[0032] In the following detailed description, for the purpose of explanation, numerous specific details are set forth in order to provide a thorough understanding of the disclosed embodiments. However, it will be apparent that one or more embodiments may be practiced without these specific details. In other cases, well-known structures and devices are schematically shown in order to simplify the drawings.

[0033] As used herein, the "intersection line" between a leg and a foot shall refer to the intersection of the plane in which the foot lies and the tangent plane at the free end of the leg. If the leg is flat and planar, the tangent plane of the leg is the plane in which the leg lies. In some cases, the "intersection line" between the leg and the foot is also the line where the foot meets the leg or the line where the leg connects to the base.

[0034] As used herein, the "free end" of a leg shall refer to the other end of the leg other than the end of the leg connected to the main board of the busbar.

[0035] As used herein, the "longitudinal direction" of the main board shall refer to the direction of the main board with the greatest extension. In most cases where the main board is generally rectangular in shape, the longitudinal direction is the direction of the long sides of the rectangle, along which one or two rows of legs are arranged.

[0036] Figure 1A typical single-layer metal busbar 1 commonly seen in the prior art is shown, which has six legs 20 and six feet 30, and an external connector 40. The busbar 1 includes a main board 10 of rectangular shape, six legs 20 (only four of which are shown in the figure) extending from two opposite sides of the main board 10, and an external connector 40 extending from one of the remaining two sides of the main board 10. Each leg 20 terminates with a foot 30 at its free end. The six legs 20 extend in a direction transverse to the face of the main board 10, and each foot 30 extends in a direction transverse to the extension direction of the corresponding leg 20. The intersection line 50 between the leg 20 and the associated foot 30 is parallel to the longitudinal direction X of the main board. The busbar 1 is formed by stamping and bending from a sheet of metal such as copper, aluminum or other relatively low-resistance metal.

[0037] As the temperature of the busbar 1 increases, the maximum linear expansion will be along the length of the structure, i.e., the longitudinal direction X, because the linearity is greater. Figure 1 The busbar 1 (not shown) will not expand much, so the mismatch between the expansion of the busbar 1 and the expansion of the substrate will cause stress at the foot 30 of each leg 20 of the busbar 1. This stress may in turn cause cracking at these interfaces and eventually lead to disconnection.

[0038] Figure 2 Shown from Figure 1 Detail of one of the legs of a busbar, which is connected to an electrical contact of the substrate. As shown, the leg 30 is formed by bending the end of the leg 20 and extends in a direction transverse to the extension direction of the leg 20. The expansion of the busbar causes a movement relative to the substrate in the direction indicated by the arrow. It has been found in a specific model that in this case, a displacement of 30 pm at the top end of the leg 20 results in a stress of 65 MPa to 70 MPa at the interface with the surface of the substrate.

[0039] Figure 3 is a view of a foot 230 of a busbar according to the present invention, and Figure 2 Compared to the legs shown in FIG. 1 , the legs 220 are twisted by approximately 90°. Figure 2 The legs shown are accordingly rotated by approximately 90°. As shown, the intersection line L between the legs 220 and the legs 230 forms an offset angle α relative to the longitudinal direction X of the main board (not shown in this figure). Figure 2 Based on modeling, it was found that a 30 pm displacement at the tip of the leg 220 would result in 25 MPa to 30 MPa at the interface with the surface of the substrate.

[0040] Thus, the stresses generated at the interface are significantly reduced by rotating the legs 230. This is because some of the movement caused by the expansion of the busbar relative to the substrate has been absorbed by the twisted structure of the legs 220.

[0041] Figure 4 is similar to Figure 1 A perspective view of a busbar 2 according to the invention is shown, with Figure 1 Compared to the legs shown, the legs 220 are twisted by about 90°. All six legs 230 (only four of which are shown in the figure) are arranged relative to each other. Figure 2 The legs shown are correspondingly rotated by about 90° in the same direction. Of course, the rotated legs 230 should be in such a position and orientation to ensure proper contact between the busbar 2 and the substrate. In this case, the intersection line L between the leg 220 and the associated leg 230 forms an offset angle of about 90° relative to the longitudinal direction X of the main board 210.

[0042] Figure 5 An alternative form of a single-layer metal busbar 3 known in the art is shown, which has five legs 320 and five feet 330, and an external connector 340. The busbar 3 comprises an elongated main board 310 in the shape of a strip, five legs 320 extending from a long side of the main board 310, and an external connector 340 extending from another long side of the main board 310. Each leg 320 terminates with a foot 330 at its free end. The five legs 320 extend in a direction transverse to the longitudinal direction of the main board 310, and each foot 330 extends in a direction transverse to the extension direction of the corresponding leg 320. The intersection line 350 between the legs 320 and the associated feet 330 is parallel to the longitudinal direction X of the main board 310. Again, the busbar 3 is formed by stamping and bending from a sheet of metal such as copper, aluminum or other relatively low resistance metal.

[0043] Figure 6 is similar to Figure 5 A perspective view of a busbar 4 according to the invention is shown, with Figure 5 Compared to the legs shown, the legs 420 are twisted by about 90°. All five legs 430 are twisted relative to Figure 5 The legs shown are correspondingly rotated by about 90° in the same direction. Of course, the rotated legs 430 should be in such a position and orientation to ensure proper contact between the busbar 4 and the substrate. In this case, the intersection line L between the leg 420 and the associated leg 430 forms an offset angle of about 90° relative to the longitudinal direction X of the main board.

[0044] Figure 7 Is caused by Figure 4A perspective view of a double-layer busbar system 5 formed by two busbars with similar busbars and rotated legs is shown. Three legs of the upper busbar and two legs of the lower busbar are visible. All five legs are rotated by approximately 90° in the same direction relative to the legs that would otherwise be formed by simply bending the end portions of the corresponding legs.

[0045] It is often advantageous to form a multi-layer busbar structure, such as for the positive conductors, negative conductors, and AC conductors of a power converter module. Figure 7 The multilayer busbar system 5 shown can be formed by placing two single-layer busbars in close proximity with an air gap or insulating film between them, or by firmly laminating two plate-like busbars together with an epoxy or thermoplastic insulating material arranged between them. Since the busbar structure itself cannot bend in this case, the more rigid the busbar structure becomes, the more severe the disconnection problem due to thermal stress at the legs becomes.

[0046] Figure 8 is a partial cutaway perspective view of a power module 8, in which a Figure 7 The power module 8 comprises a base plate 810 on which is placed a DBC substrate 820 which exhibits (conductive) tracks 830 on its top surface on which semiconductors 840 are mounted. There may also be other wiring such as wire bonds to connect the semiconductors, which are not shown in the figure. The double-layer busbar system 5 with two external connectors 540 connected to the tracks 830 on the DBC substrate 820 via rotating feet. A frame 850 through which the external connectors 540 protrude is assembled on top of the base plate 810 and surrounds the DBC 820, the semiconductors 840 and the busbar system 5. A cover 860 on top of the frame 850 closes the power module 8.

[0047] Fig. 9 is a plan view looking down on the leg structure and showing three possible positions of the rotated leg 30, and Fig.10 is a plan view looking down on the leg structure and illustrating two possible positions of the rotated foot 30. Fig. 9 Position A and position B shown represent a rotation of 90° from the original position. Fig. 9 The position C shown represents a rotation of 180°. Such rotation need not be limited to integer multiples of 90°. For example, Fig.10 Two possible rotations (positions A' and B') of less than 90° (eg 45°) are shown.

[0048] Fig.111 is a flow chart showing the steps of a method for forming a busbar according to the present invention. First, a planar structure having a main board and at least one leg is punched or cut 1101 from a conductive sheet. Then, the at least one leg is bent 1102 out of the plane of the main board, and the end portion of the at least one leg is bent to form a foot. Subsequently, the at least one leg is twisted 1103 in such a way that the intersection line between the at least one leg and the associated foot forms a certain offset angle relative to the longitudinal direction of the main board. It should be understood that the order of the last two steps can be reversed. Further, the foot can be formed by bending the end portion of the leg before bending the leg out of the plane of the main board.

[0049] like Figure 1 and Figure 5 The busbars shown are typically made by stamping or cutting a shape from a flat sheet and then bending the shape to form the legs, feet and external contacts. Such structures can also be made by additive manufacturing techniques, by casting or by any other known technique suitable for the purpose. The formation of the twist in the legs can be achieved by a fairly simple extension of casting or additive manufacturing, but it can also be achieved by physically twisting the flat sheet of the busbar described above.

[0050] Although several exemplary embodiments have been shown and described, it will be understood by those skilled in the art that various changes or modifications may be made to these embodiments without departing from the principles and spirit of the present disclosure, the scope of which is defined in the claims and their equivalents.

[0051] As used herein, elements recited in the singular and preceded by the word "a" or "an" should be understood as not excluding plural said elements or steps, unless such exclusion is explicitly stated. Furthermore, references to "one embodiment" of the present invention are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features.

Claims

1. A conductive busbar (2, 4) suitable for a semiconductor power module (8), the conductive busbar comprising: A main board (210, 410), one or more legs (220, 420) extending from the main board (210, 410), and one or more feet (230, 430) formed at the free ends of the legs (220, 420), wherein an intersection line (L) between at least one of the legs (220, 420) and an associated foot (230, 430) forms an offset angle (α) relative to a longitudinal direction (X) of the main board (210, 410), wherein at least one of the legs (220, 420) of the busbar (2, 4) includes a twisted portion, and the plane of the leg (220, 420) is rotated by the twisted portion by the offset angle (α).

2. The conductive busbar (2, 4) according to claim 1, wherein: The offset angle (α) formed by the intersection line (L) between the at least one of the legs (220, 420) and the associated foot (230, 430) relative to the longitudinal direction (X) of the main board (210, 410) is from 3° up to 180°.

3. The conductive busbar (2, 4) according to claim 2, wherein: The offset angle (α) is 45° to 135°.

4. The conductive busbar (2, 4) according to claim 3, wherein: The offset angle (α) is 90°.

5. The conductive busbar (2, 4) according to claim 1, wherein: Each leg (220, 420) of the busbar (2, 4) comprises a twisted portion, through which the plane of the leg (220, 420) is rotated by an offset angle (α).

6. The conductive busbar (2, 4) according to claim 5, wherein: All of these legs (220, 420) are twisted at the same angle and in the same direction.

7. The conductive busbar (2, 4) according to any one of claims 1 to 6, wherein: The legs (220, 420) of the busbar (2, 4) extend from one side or two opposite sides of the main board (210, 410) of the busbar (2, 4).

8. The conductive busbar (2, 4) according to claim 7, wherein: The legs (220, 420) of the busbar (2, 4) extend from two longitudinal edges of the main board (210, 410) of the busbar (2, 4).

9. A busbar system (5), comprising two or more busbars (2, 4) according to any one of claims 1 to 8, wherein: The busbars (2, 4) are in close proximity with an air gap or an insulating film or layer between them.

10. A power module (8), comprising the busbar (2, 4) according to any one of claims 1 to 8 or the busbar system (5) according to claim 9.

11. A method for forming a conductive busbar (2, 4), the method comprising the following steps: Punching or cutting (1101) a planar structure having a main board (210, 410) and at least one leg (220, 420) from a conductive sheet; Bend (1102) the at least one leg (220, 420) out of the plane of the main board (210, 410), and bend an end portion of the at least one leg (220, 420) to form a foot (230, 430); as well as The at least one leg (220, 420) is twisted (1103) in such a way that an intersection line (L) between the at least one leg (220, 420) and an associated foot (230, 430) forms an offset angle (α) relative to a longitudinal direction (X) of the main board (210, 410).

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